It is known that combustion by compressed self-ignition, in which a mixture gas combusts instantly without flame propagation, maximizes fuel efficiency since the combustion period is minimal. However, various problems must be solved for automobile engines with regard to combustion by compressed self-ignition. For example, since the operating states and the environmental conditions vary greatly in automotive applications, stabilizing compressed self-ignition is a major problem. The combustion by compressed self-ignition has not been put to practical use for the automobile engine yet. In order to solve the problem, for example, JP4,082,292B2 proposes that an ignition plug ignites the mixture gas, when compressed self-ignition hardly occurs because of a low combustion-chamber temperature. By igniting the mixture gas immediately before the compression top dead center, the pressure around the ignition plug increases to facilitate the compressed self-ignition.
Unlike the technology disclosed in JP4,082,292B2 in which the compressed self-ignition is assisted by the ignition of the ignition plug, the present applicant rather proposes SPCCI (SPark Controlled Compression Ignition) combustion which is a combination of SI (Spark Ignition) combustion and CI (Compression Ignition) combustion. The SI combustion is combustion accompanied by the flame propagation initiated by forcibly igniting the mixture gas inside the combustion chamber. The CI combustion is combustion initiated by the mixture gas inside the combustion chamber carrying out the compressed self-ignition. The SPCCI combustion is combustion in which, when the mixture gas inside the combustion chamber is forcibly ignited to start the combustion by flame propagation, the unburnt mixture gas inside the combustion chamber combusts by the compression-ignition because of a pressure buildup due to the heat generation and the flame propagation of the SI combustion. Since the SPCCI combustion includes the CI combustion, it is one form of “the combustion by compression-ignition.”
The CI combustion takes place, when the in-cylinder temperature reaches an ignition temperature defined by the composition of the mixture gas. Fuel efficiency can be maximized, if the in-cylinder temperature reaches the ignition temperature near a compression top dead center and the CI combustion takes place. The in-cylinder temperature increases according to the increase in the in-cylinder pressure. The in-cylinder pressure in the SPCCI combustion is a result of two pressure buildups of a pressure buildup by the compression work of the piston in a compression stroke, and a pressure buildup caused by the heat generation of the SI combustion.
Here, if the CI combustion takes place near a compression top dead center because of a high in-cylinder temperature at a compression starting timing due to a high ambient temperature, etc., the in-cylinder pressure excessively increases to create excessive combustion noise. In this case, combustion noise can be reduced if the ignition timing is retarded. However, if the ignition timing is retarded, since the CI combustion takes place when the piston falls considerably in the expansion stroke, fuel efficiency is decreased. Since the pressure buildup caused by the heat generation of the SI combustion can be utilized in the SPCCI combustion, for example, it is effective to lower the effective compression ratio and to reduce the pressure buildup by the compression work of the piston in order to achieve both the reduction of combustion noise and improvement in fuel efficiency. Thus, combustion noise can be kept suitable, without decreasing fuel efficiency.
In order to put to practical use the engine which performs the SPCCI combustion, it is necessary to take into consideration other control factors relevant to the in-cylinder temperature, other than effective compression ratio. However, since the SPCCI combustion is a new combustion system, no one has found other control factors until now.
Since the SPCCI combustion is compression-ignition combustion, combustion noise tends to be increased. The present inventors found that it was necessary to adjust the temperature inside the combustion chamber at the start timing of the CI combustion to a suitable temperature, in order to achieve a stable SPCCI combustion, while reducing combustion noise. If the temperature inside the combustion chamber is low, the ignitability of the CI combustion falls. Combustion noise increases as the temperature inside the combustion chamber goes up.
The temperature inside the combustion chamber mainly depends on the geometric compression ratio of the engine, and the temperature and/or amount of gas introduced into the combustion chamber. Properties related to the gas introduced into the combustion chamber depend on the supercharging pressure of the supercharger, and the valve timing of the intake valve, especially when boosting. The optimal supercharging pressure was unknown so far.